1. Field of the Invention
The present invention relates to a registration accuracy measurement mark, and more particularly, to a structure of a registration accuracy measurement mark.
2. Description of the Background Art
In recent years, the registration accuracy of layers in a device formed of a plurality of layers has become more critical in accordance with miniaturization of the dimension of the device. A registration accuracy measurement mark used for the purpose of assessing the registration accuracy will be described hereinafter for an MOS transistor.
FIG. 22 shows a vertical sectional view of a general MOS transistor, and FIG. 23 shows a plan view of a semiconductor device in which such an MOS transistor is provided. The structure of an MOS transistor will be briefly described with reference to FIGS. 22 and 23. On a semiconductor substrate 106, a word line 110a serving as a gate electrode is formed with a gate oxide film 108 thereunder. Source/drain regions 107 are formed at semiconductor substrate 106.
A bit line 112A is provided above gate electrode 110A with an interlayer oxide film 110 therebetween. Bit line 112A is electrically connected to one of source/drain regions 107. Word line 110A and bit line 112A are arranged so as to be at right angles to each other as shown in FIG. 23. An interlayer oxide film 113 is formed on bit line 112A.
Formation of a contact hole 104 in an active region 115 between word lines 110A and bit lines 112A disposed at intervals of 1 .mu.m respectively in a semiconductor device of the above structure will be described hereinafter with reference to FIG. 23.
The dimension of contact hole 104 formed in the semiconductor device is 0.5 .mu.m.times.0.5 .mu.m. When word line 110A, bit line 112A and contact hole 104 are formed in accurate registration as designed, the distance of X between word line 110A and contact hole 104 in the X direction and the distance of Y between bit line 112A and contact hole 104 in the Y direction are both 0.25 .mu.m.
However, contact hole 104 may be formed offset in position in some cases. When there is offset in position, a portion of contact hole 104 will be formed on bit line 110A and bit line 112A.
Contact hole 104 is formed including the steps of patterning a resist film 114A formed on interlayer oxide film 113 by lithography, and providing a contact hole using the patterned resist film 114A as a mask. At the stage of patterning resist film 114A, the offset in position of the contact hole pattern formed by resist film 114A with respect to the positions of word lines 110A and bit line 112A can be measured, and then reproduce only the resist film if the contact hole pattern of the resist film is not formed accurately.
However, since the distance between contact hole 104 and word line 110A, and the distance between contact hole 104 and bit line 112A are as small as 0.25 .mu.m, it was extremely difficult to measure the registration accuracy of this region.
According to a conventional method of measuring the registration accuracy of the contact hole pattern of a resist film with respect to a word line or a bit line, a registration accuracy measurement mark is provided in the periphery of the region where a semiconductor device is formed as a dummy pattern for measuring the accuracy in the region. This registration accuracy measurement mark is formed simultaneous to the formation process of a word line, a bit line, and a resist film. The registration accuracy is assessed on the basis of this registration accuracy measurement mark.
This registration accuracy measurement mark will be described hereinafter with reference to FIGS. 24-26. First, the arrangement of a registration accuracy measurement mark will be described with reference to FIG. 24. In a peripheral region of a semiconductor device, a first measurement mark 110B is formed at a predetermined position on a gate oxide film 108 simultaneous to the formation step of word line 110A. The plane configuration of first measurement mark 110B has a square pattern of 20 .mu.m.times.20 .mu.m as shown in FIG. 25(a). Also, a second measurement mark 112B is formed at a predetermined position on an interlayer oxide film 110 simultaneous to the step of forming bit line 112A. The plane configuration of second measurement mark 112B has a square pattern of 20 .mu.m.times.20 .mu.m as shown in FIG. 26(a).
Above first measurement mark 110B and second measurement mark 112B, third and fourth measurement marks 114B and 114C are formed on interlayer insulation film 113 simultaneous to the respective patterning steps of respective resist films.
Third and fourth measurement marks 114B and 114C have a square pattern of 10 .mu.m.times.10 .mu.m as shown in FIGS. 25(a) and 26(a), respectively.
Measurement of registration accuracy using first, second, third, and fourth measurement marks 110B, 112B, 114B, and 114C, respectively, will be described with reference to FIGS. 25 and 26.
FIG. 25 relates to the registration accuracy between word line 110A and the contact hole pattern of a resist film. It is appreciated from FIG. 23 that the registration accuracy between word line 110A and contact hole pattern is crucial in the X direction.
In FIG. 25, (a) shows a plan view of third measurement mark 114B, and (b) shows the intensity of light by a detection signal taken along lines A-A' of (a) when light is directed to third measurement mark 114B. It is appreciated from FIG. 25 that intensity of light according to a detection signal is low at positions corresponding to sidewalls 10a, 10b, 11a, and 11b of first and third measurement marks 110B and 114B. The registration accuracy in the X direction is measured according to this detection signal.
For example, a middle point c.sub.1 of the detection signals of sidewalls 10a and 10b, and a middle point c.sub.2 of detection signals of sidewalls 11a and 11b are obtained. When the positions of middle points c.sub.1 and c.sub.2 match, the offset between first registration mark 110B and third registration mark 114B in the X direction is 0. An offset between the positions of middle points c.sub.1 and c.sub.2, if any, corresponds to the amount of offset between first and third measurement marks 110B and 114B in the X direction. Similarly, the amount of offset in the Y direction between second and fourth measurement marks 112B and 114C is obtained as shown in FIGS. 26(a) and (b).
The measured results using first, second, third and fourth measurement marks 110B, 112B, 114B, and 114C have a one to one correspondence with respect to the amount of offset of word line 110A, bit line 112A, and the contact hole pattern of the resist film. They can be taken directly as the registration accuracy.
The above-described measuring method using a registration accuracy marks requires the provision of two types of measurement marks at different positions, i.e. a measurement mark for measuring the registration accuracy in the X direction, and a measurement mark for measuring the registration accuracy in the Y direction. This means that a region for forming this measurement mark is necessary.
When the registration accuracy is measured using light, individual measurement steps must be carried out since the layers subject to measurement in the X direction and measurement in the Y direction differs. The measurement process is time consuming, and becomes a bottle neck in reducing the overall manufacturing time required for a semiconductor device.